Fantailer is right, you need a way larger tank. To save your fish you should relocate them ASAP! They are poisoning themselves with their own waste and will not live much longer unless you do something fast. It is recommended to do weekly 50% water changes on a suitable sized tank. Testing the water will also help you to maintain a safe tank when you set up the new one. Also make sure you aren't over-feeding as that can cause even more ammonia from uneaten food.